Team fortress crashing...

i just built this computer and its awesome, but its crashing... only on team fortress when i play for a while, i have a intel quad core 2.4ghz ,640mb 8800gts, 2gb pc 6400 ram , liquid cooled, 600 w att powersupply, and a 7200 rpm HDD, i have the settings completely maxed, i mean everything, runs great! but after like 30 minutes it crashed, you guys think i just need to upgrade to 4gb of ram? im running vista and it says its i ave 1.4 gb of video ram, its sharing over 700 mb of my system ram or video?? any help would be nice thanks!